Pyrococcus furiosus ferredoxin is unique among ferredoxins in having a single Fe 4S4 cluster with one iron possessing non-sulfur coordination. An unusual feature of the protein is the ease with which the cluster can lose this iron to form a Fe 3S4 ferredoxin. The Fe 3S4 ferredoxin can readily be reconstituted with Fe to reform the native Fe4 S4 cluster, or alternatively with other metals ions to generate a mixed-metal iron sulfur cluster MFe3S4. Mixed metal iron sulfur clusters comprise a very important class of metalloprotein active sites and are of considerable interest. We propose to use XAS to systematically probe the electronic and physical structure of the different MFe 3S4 clusters in Pyrococcus furiosus ferredoxin.